Why would you know that?

Arthur Krystal’s profile of Jacques Barzun in The New Yorker, on the occasion of his hundredth birthday:

Sooner or later, all of Barzun’s acquaintances experience their own “just Jacques” moment. Two years ago, while working on a piece for this magazine, I called Barzun to find out whether Lord Edward Grey, the British Foreign Secretary during the First World War, had said that the lights were going out all over Europe before hostilities had actually begun. Barzun asked if I was referring to him in my article as “Lord Grey.” I said I was, since the attribution was always the same. Barzun cleared his throat. “Well, you know, he wasn’t a lord when he said it. He didn’t become Viscount of Fallodon until 1916.” For the first time in thirty-odd years of conversation, I exclaimed, “Why would you know that?” He replied, mildly, “It’s my business to know such things.”